One known cooling solution consists in providing the turbine ring with multiply-perforated metal sheets that surround said ring. A solution of that type is described, for example in document EP 0 893 577. FIG. 1 shows an example of a ring sector analogous to that described in document EP 0 893 577, in which a turbine ring sector 6 has a wall 5 defining an airflow passage that is axially oriented along the axis X along which the gas flows. A multiply-perforated metal sheet 1 is situated on the side of the wall 5 that is opposite from the airflow passage. Said multiply-perforated metal sheet 1 comprises a bottom 2 and side walls 3. The (empty) space between the wall 5 of the ring sector 6 and the bottom 2 of the multiply-perforated metal sheet 1 defines a gap E.
In such a system, the axial direction is defined by the axis of rotation of the rotor blades. A radial direction is defined by a radius of a disk perpendicular to the axis of rotation of the rotor blades and centered on said axis. The circumferential direction is the direction that is tangential to the tips of the rotor blades when they are in rotation. These three directions (axial, radial, and circumferential) define a system of axes of the cylindrical type. Furthermore, in this example, “upstream” and “downstream” are defined relative to the flow direction (from upstream to downstream) of cooling gas through the multiply-perforated metal sheet 1 (flow that is directed towards the wall 5 of the ring sector 6).
In this prior art example, the gap E is constant. Perforations 4 are distributed over the bottom 2 of the multiply-perforated metal sheet 1. The perforations that are situated in the bottom 2 in its center, in the axial direction, are referenced 4C0 on the outside (their immediate neighbors in the axial direction) are referenced 4C1. The perforations bordering the perforations 4C1 on the outside (their outer immediate neighbors in the axial direction) are referenced 4C2, etc.
FIG. 1A is a diagram showing the air streams through the FIG. 1 ring sector. Cooling gas passes through the metal sheet 1 via the perforations 4 in the form of a radial stream of gas, and impacts the wall 5 of the ring sector 6 in order to cool it. After impacting the wall 5, the gas is deflected towards the sides C of the wall 5. Thus, the radial stream F0 of gas that passes through the central perforations 4C0 gives rise to a stream f0 that is oriented in the axial direction. This axial stream f0 of gas (deflected radial stream F0) shears the radial stream F1 of gas leaving the perforations 4C1. The efficiency of the impact cooling due (solely) to the radial stream F1 of gas coming from the perforations 4C1 is thus reduced. In the same manner, the radial stream F1 of gas coming from the perforations 4C1 is deflected in the axial direction by the wall 5, and gives rise to an axial stream f1 that adds to the axial stream f0 of gas (deflected radial stream F0) coming from the perforations 4C0, and shears the radial stream F2 of gas coming from the perforations 4C2, and so on. Thus, the radial stream Fn (where n is an integer greater than or equal to 1) of gas coming from a peripheral perforation 4Cn impacting the wall 5 is sheared by the axial stream of gas resulting from the wall 5 deflecting the radial streams F0, F1, . . . , Fn−1 coming from the perforations situated closer to the center 4C0, 4C1, . . . , 4Cn−1. The overall efficiency of the cooling of the wall 5 by impact is thus reduced on going away from the center of the bottom 2 of the metal sheet 1. The cooling of the ring sector is thus not uniform, the ring sector 6 being better cooled in its central region than at its periphery (in the axial direction).